While leveling up, any instance can be healed by any spec, assuming decent gear. It's not always easy, but it's possible. Being a healing spec would make the healer more effective, so how can that be bad? It teaches a false sense of security and power.

When someone makes a bad pull in a lowbie instance and have a shadow priest healing, they might die. Do the same with a holy priest and they're more likely to live. The more effective healer shelters people from the consequences of their actions. No deaths, no learning.

On the healer's side, it's about as bad. They are more efficient and heal for more. There is very little challenge. What will happen when they get to heroics or raids where being specced for healing isn't just an advantage that trivializes the content, but a requirement to even stand a chance? Healing while leveling is easy enough already, doing it with a healing spec is overkill.

Finally there's just the fact that healing specs kill slower, so they're inferior for soloing. This makes leveling take longer and it can become frustrating. That results in more alts stuck at low levels with the player turned off to the class.

3
comments:

I would almost agree with you except the changes from +heal to spellpower would put some bite into the burst damage capabilities.

I can't comment on other classes, but I have played many a paladin, including full holy spec. Would I level holy? No. Is it viable? I believe so, with JoW refreshing the mana pool and holy shock thrown in when the cooldown is up, I believe you could get a lot of burst damage. Not the efficiency of prot, or the overall damage of ret, but you're a healer.

Simple -- if you're a priest, you make a discipline/holy priest hybrid build. You give yourself enough healy talents to get the job done but spec for self protection and increased damage, enough that soloing really isn't an issue. If anything I think it makes for a better healer at 80 -- as you're coming into healing with 'being flexible' and 'self preservation' already in mind.

Verses the frail little floundering priest you sometimes get in end game, where they're reflexes are dull and they're wimpering at their tank who lost aggro 'help, help get it off me...'